How to allocate array size in Python [duplicate]

Question

Python newbie here. I've searched quite a bit for a solution to this but nothing quite fits what I need. I would like to allocate an empty array at the start of my program that has a rows and b columns. I came up with a solution but encountered an interesting problem that I didn't expect. Here's what I had:

a = 7
b = 5
array_ab = [['?'] * b] * a

which produces

[['?', '?', '?', '?', '?'],
 ['?', '?', '?', '?', '?'],
 ['?', '?', '?', '?', '?'],
 ['?', '?', '?', '?', '?'],
 ['?', '?', '?', '?', '?'],
 ['?', '?', '?', '?', '?'],
 ['?', '?', '?', '?', '?']]

However, if I try to change a single element, it treats every row as the same object and effectively changes the entire column to that element. So for example

array_ab[4][2] = '1'

produces

[['?', '?', '1', '?', '?'],
 ['?', '?', '1', '?', '?'],
 ['?', '?', '1', '?', '?'],
 ['?', '?', '1', '?', '?'],
 ['?', '?', '1', '?', '?'],
 ['?', '?', '1', '?', '?'],
 ['?', '?', '1', '?', '?']]

Clearly I need a better way to create the blank array than by multiplication. Is there a solution to this in python? (It was so simple in FORTRAN!)

Answer 1

Something along the lines of

In [12]: a = 5

In [13]: b = 7

In [14]: array_ab = [ [ '?' for i in xrange(a) ] for j in xrange(b) ]

In [15]: array_ab
Out[15]:
[['?', '?', '?', '?', '?'],
 ['?', '?', '?', '?', '?'],
 ['?', '?', '?', '?', '?'],
 ['?', '?', '?', '?', '?'],
 ['?', '?', '?', '?', '?'],
 ['?', '?', '?', '?', '?'],
 ['?', '?', '?', '?', '?']]

In [16]: array_ab[4][2] = '1'

In [17]: array_ab
Out[17]:
[['?', '?', '?', '?', '?'],
 ['?', '?', '?', '?', '?'],
 ['?', '?', '?', '?', '?'],
 ['?', '?', '?', '?', '?'],
 ['?', '?', '1', '?', '?'],
 ['?', '?', '?', '?', '?'],
 ['?', '?', '?', '?', '?']]

In particular, you're using list comprehensions and xrange.

Answer 2

If you are going to use your array for numerical computations, and can live with importing an external library, then I would suggest looking at numpy. It provides an array class and lots of useful array operations.

Creating an MxN array is simply

import numpy as np

A = np.empty((M,N)) # Empty array
B = np.zeros((M,N)) # Array filled with zeros

Indexing is then done like

x = A[i,j]
A[4,2] = 1

row1 = A[0, :] # or simply A[0]